Datasheet

Level
Shift
Internal
Circuitry
+3.3V
+3.3V
800kW
R
PU
Input
Pin
Input
C
0.1 Fm
IN
V =V
DC S
800kW
800k +RW
PU
THS7360
SLOS674 –JUNE 2010 www.ti.com
It should be noted that this STC system is dynamic there is a loss of sync occurring, then simply
and does not rely upon timing in any way. It only decrease the 0.1-mF input coupling capacitor. A
depends on the voltage that appears at the input pin decrease from 0.1 mF to 0.047 mF increases the hum
at any given point in time. The STC filtering helps rejection by a factor of 2.1. Alternatively, an external
minimize level shift problems associated with pull-down resistor to ground may be added that
switching noises or very short spikes on the signal decreases the overall resistance and ultimately
line. This architecture helps ensure a very robust increases the discharge current.
STC system.
To ensure proper stability of the ac STC control loop,
When the ac STC operation is used, there must also the source impedance must be less than 1-kΩ with
be some finite amount of discharge bias current. As the input capacitor in place. Otherwise, there is a
previously described, if the input signal goes below possibility of the control loop ringing, which may
the 0-V clamp level, the internal loop of the THS7360 appear on the output of the THS7360. Because most
sources current to increase the voltage appearing at DACs or encoders use resistors to establish the
the input pin. As the difference between the signal voltage, which are typically less than 300-Ω, meeting
level and the 0-V reference level increases, the the less than 1-kΩ requirement is easily done.
amount of source current increases However, if the source impedance looking from the
proportionally—supplying up to 6 mA of current. THS7360 input perspective is very high, then simply
Thus, the time to re-establish the proper STC voltage adding a 1-kΩ resistor to GND ensures proper
can be very fast. If the difference is very small, then operation of the THS7360.
the source current is also very small to account for
minor voltage droop.
INPUT MODE OF OPERATION: AC BIAS
However, what happens if the input signal goes
Sync-tip clamps work very well for signals that have
above the 0-V input level? The problem is that the
horizontal and/or vertical syncs associated with them;
video signal is always above this level and must not
however, some video signals do not have a sync
be altered in any way. As a result, if the sync level of
embedded within the signal. If ac-coupling of these
the input signal is above this 0-V level, then the
signals is desired, then a dc bias is required to
internal discharge (sink) current reduces the
properly set the dc operating point within the
ac-coupled bias signal to the proper 0-V level.
THS7360. This function is easily accomplished with
the THS7360 by simply adding an external pull-up
This discharge current must not be large enough to
resistor to the positive power supply, as shown in
alter the video signal appreciably or picture quality
Figure 46.
issues may arise. This effect is often seen by looking
at the tilt (droop) of a constant luma signal being
applied and the resulting output level. The associated
change in luma level from the beginning and end of
the video line is the amount of line tilt (droop).
If the discharge current is very small, the amount of
tilt is very low, which is a generally a good thing.
However, the amount of time for the system to
capture the sync signal could be too long. This effect
is also termed hum rejection. Hum arises from the ac
line voltage frequency of 50 Hz or 60 Hz. The value
of the discharge current and the ac-coupling capacitor
combine to dictate the hum rejection and the amount
of line tilt.
Figure 46. AC-Bias Input Mode Circuit
To allow for both dc- and ac-coupling in the same
Configuration
part, the THS7360 incorporates an 800-kΩ resistor to
ground. Although a true constant current sink is
The dc voltage appearing at the input pin is equal to
preferred over a resistor, there can be issues when
Equation 1:
the voltage is near ground. This configuration can
cause the current sink transistor to saturate and
cause potential problems with the signal. The 800-kΩ
(1)
resistor is large enough to not impact a dc-coupled
DAC termination. For discharging an ac-coupled
source, Ohm’s Law is used. If the video signal is
0.5 V, then there is 0.5 V/800 kΩ = 0.625-mA of
discharge current. If more hum rejection is desired or
26 Submit Documentation Feedback Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): THS7360